It has been found that all conventional power operated tape measures on the market suffer from a lot of drawbacks thereby rendering them unfit for practical use.
FIG. 1 shows a first prior art power-operated tape measure. As illustrated, the power operated tape measure includes a motor A which is engaged with the internal threads C1 of a reel C via a gear train B. The external threads C2 of the reel C is meshed with a driving gear E1 which is connected with a shift rod F and engaged with a driven gear E2. A roller E3 is disposed under the driven gear so as to form a passage for a tape D. When desired to push the tape D out of the case, it is only necessary to push the button G to its OUT position. Similarly, it is only necessary to push the button G to its IN position when desired to withdraw the tape D. However, the tape D cannot be steadily pulled into the case and furthermore such a power tape measure is too complicated in structure.
FIGS. 2A and 2B show the structure of the tape measure disclosed in U.S. Pat. No. 4,186,490. In this patent, the reel is pushed by four spring-loaded rollers A1 and the tape is formed with slots engaged between two toothed rollers P and Q. Nevertheless, the slots will weaken the strength of the tape and the two toothed gears will increase the manufacturing cost hence making it difficult to be accepted by the user.
Similar structures of the tape measure are shown in FIGS. 3A and 3B (U.S. Pat. No. 5,044,089) and FIGS. 4A and 4B (U.S. Pat. No. 4,551,347). Nonetheless, both two patents suggest forming slots in the tape and so they still suffer from the similar drawbacks of the previous discussed prior art tape measure.
Therefore, it is an object of the present invention to provide a power-driven tape measure which can obviate and mitigate the above-mentioned drawbacks.